1. Field of the Invention
The present invention relates to an image reading apparatus used together with an image forming apparatus and an image reading apparatus used singly.
2. Description of the Related Art
In general, an image reading apparatus mounted on a digital copying machine or the like includes an imaging lens, a line sensor, and a reflecting mirror (Japanese Patent Laid-Open No. 3-113961). The imaging lens and the line sensor are fixed in a housing. On the other hand, the reflecting mirror is mounted on a movable scanning unit to move in the sub-scanning direction with respect to an original. In the image reading apparatus described in Japanese Patent Laid-Open No. 3-113961, the maximum angle of view is generally designed to be almost 20°.
FIG. 23 is a plan view for explaining the angle of view in an image reading apparatus. The vertical direction of FIG. 23 corresponds to the main-scanning direction of the image reading apparatus. The horizontal direction of FIG. 23 corresponds to the sub-scanning direction. The reflection angle of an original image reflected by a reflecting mirror 522 changes between an end portion and the central portion in the main-scanning direction. More specifically, at each end portion in the main-scanning direction, the reflecting mirror 522 reflects light at a predetermined angle θ to condense it onto an imaging lens 525. The angle θ is called an angle of view. The angle of view gradually becomes smaller toward the center in the main-scanning direction. Especially, the angle θ of view is 0° at the center in the main-scanning direction. That is, the angle of view of the original image changes depending on the position where the reflecting mirror 522 reflects the light.
Recently, size reduction of image reading apparatuses has received a great deal of attention. Japanese Patent Laid-Open No. 2004-126448 has proposed an image reading apparatus which reduces the size by employing, as an imaging element, an off axial imaging unit which images via a plurality of mirrors each having an off axial reflecting surface. An off axial reflecting surface is a reflecting surface having a curvature and a reflecting direction different from the reference optical axis line incident direction.
However, an image reading apparatus needs shading correction to correct light amount unevenness independently of its scale. In general, a color image reading apparatus has, on the photoelectric conversion elements of a line sensor, three color filters to pass red (R), green (G), and blue (B) light components, respectively. An RGB line sensor receives the light components that have passed through the three color filters and photoelectrically converts them, thereby obtaining RGB read luminance signals. The illuminance of a light source generally varies. In addition, the light amount around the imaging lens or the imaging mirror decreases. For these reasons, unevenness (shading) occurs in the illuminance on the imaging plane, and shading correction is necessary.
In shading correction, generally, the sensor reads a white reference member immediately before reading an original. Based on the reading result, the gain and offset are adjusted for each pixel.
However, shading correction using the white reference member is possible only for light amount variations which are not related to the spectral characteristic (spectral optical characteristic) of the reading optical system, such as the illuminance unevenness of the light source or the decrease in the light amount around the imaging lens. That is, it is impossible to correct the influence of a change in the spectral characteristic caused by the difference in the angle of view between the reflecting mirror, the imaging mirror, and the imaging lens.
FIG. 24 is a graph showing the spectral characteristics of the reflecting mirror corresponding to different angles of view. The abscissa represents the wavelength, and the ordinate represents the reflectance. As is apparent from FIG. 24, when the angle of view becomes larger, the spectral characteristic shifts toward shorter wavelengths as a whole.
The change in the spectral characteristic depends on the angle of view when an original image becomes incident on the reflecting mirror, the imaging mirror, and the imaging lens. For this reason, the larger the angle of view is, the larger the change in the spectral characteristic is. Note that the spectral characteristic of the entire reading optical system is given as the product of the spectral characteristics of all elements included in the reading optical system. At the time of shading correction, therefore, the change in the spectral characteristic of the whole optical system depending on the angle of view affects the spectral characteristic of the white reference member.
Especially when the light reflected by the original is light of a color (achromatic color such as white, black, or gray) having a spectral characteristic similar to that of white of the white reference member, the effect of shading correction is surely obtained. However, when the light reflected by the original is chromatic color light, the main-scanning read luminance becomes uneven even after shading correction. This is because the shading correction, which is performed based on light of a peak wavelength of white light, cannot completely correct shading of chromatic color light that is different from the peak wavelength. This problem can arise both in reading a chromatic color using an RGB line sensor and in reading a chromatic color using a monochrome line sensor to be described later.
In general, when the difference in the angle of view between an end portion and the central portion in the main-scanning direction is small, the influence of the change in the spectral characteristic of the optical system depending on the angle of view also becomes small. For example, as in the image reading apparatus described in Japanese Patent Laid-Open No. 3-113961, the difference in the angle of view can be made smaller by prolonging the optical path from the reflecting mirror to the CCD sensor. However, since a longer optical path leads to an increase in the size of the image reading apparatus, the object to obtain a compact apparatus cannot be achieved.
In addition to the white reference member, reference members of the respective colors such as red, green, and blue, or cyan, magenta, and yellow having managed densities may be provided, and a shading correction coefficient may be decided for each color. This method also enables to reduce the unevenness in the main-scanning read luminance of a chromatic color.
In this method, however, since the number of reference members that require density management increases, the cost increases inevitably. Additionally, a memory needs to hold shading correction coefficients equal in number to the colors of the reference members. Also required is a circuit for determining a color on an original and selecting a correction coefficient in accordance with the color. This makes the shading correction circuit larger and more complex.